This invention relates to an automatic level control system, and particularly to a chroma level control system installed in a color video tape recorder.
In a color video signal reproducing apparatus in which a magnetic tape and a rotary magnetic head are employed, signal levels of the reproduced video signal change based on tape tension, head-to-tape contacting pressure, etc. Particularly, when an NTSC type color video signal is directly converted into a FM video signal for recording, levels of the chrominance signal which occupy higher frequency regions are more influenced when reproduced.
Accordingly, it is very common to control the levels of the reproduced chrominance signals based on the comparison of a reproduced color burst signal and a reference signal.
According to a recent analysis of the level changes of the reproduced chrominance signals, there exist random level changes and standing-wave-like stationary level changes. The latter is thought to be generated cyclically based on changes of tape-head contacting pressure during one revolution of the video head. As is well-known, the video tape is run on a periphery of a tape guide drum in which the video head is installed. Usually, tape wrap angle in a helical scan video tape recorder is either 180.degree. or near 360.degree..
In a conventional level control system, these level changes are controlled by a control signal generated at a single error detecting circuit. Therefore, the design of such a control circuit which had rapid response with adequate control gain is very difficult due to the signal-to-noise ratio of the control loop.